1. Field of the Invention
The present invention relates to an extrusion molding apparatus for molding a ceramic molded product such as a honeycomb ceramic structure.
2. Description of the Related Art
For the catalyst carrier in an exhaust gas purification system of an automotive vehicle, for example, a honeycomb structure 8 is used as a ceramic molded product including a multiplicity of cells 88 having partitioning walls 81 as shown in FIG. 7. This honeycomb structure 8 is normally produced by extrusion molding.
The conventional extrusion molding apparatus 9 comprises, for example, a mold 91 for molding the honeycomb structure 8, and a screw extruder 98 for kneading and extruding a ceramic material 80 continuously, as shown in FIG. 8.
As shown in FIG. 8, a filter unit 93 for filtering the ceramic material 80 is interposed between the screw extruder 98 and the mold 91. This filter unit 93 is for preventing foreign matter from mixing with the honeycomb structure 8, and intended to trap foreign matter of a predetermined size or more by a filter unit net 930. For this purpose, the filter unit 93 includes the filter net 930 having a multiplicity of pores and a support member 935 for supporting the filter net 930. A resistance tube 92 making up a material path is normally arranged between the filter unit 93 and the mold 91.
In extrusion molding the honeycomb structure 8, the ceramic material introduced into the screw extruder 98 is filtered by the filter unit 93 while being supplied to the mold 91.
In the conventional extrusion molding apparatus 9 as described above, however, the following problem can be encountered.
Specifically, the ceramic material passing through the screw extruder is sometimes partially aggregated into a lump. The aggregated ceramic material (material lump) is trapped on the inlet surface of the filter unit 93 and reduces the fluidity of the subsequently influent ceramic material.
More specifically, as shown in FIG. 9, after the material lump 89 is trapped in the filter net 930, the ceramic material 80 passes around the material lump 89, so that the fluidity of the material is reduced while, at the same time, the flow velocity distribution of the material after the filter net is adversely affected. For this reason, various measures have been studied to supply the lumps of the ceramic material to the mold after they are restored to the original particulate form. Nevertheless, a satisfactory measure has yet to be established.
In the case where a plurality of screw extruders are provided in a plurality of stages as shown in FIG. 8, the filter unit 94 may be arranged also at the extrusion exit of the screw extruder 99 in the upper stage. A similar problem is posed in such a case.
This problem is shared by the extrusion molding apparatus used for extrusion molding various shapes of ceramic molded products such as sheets, round bars and pipes as well as honeycomb structures.
The present invention has been developed in view of the problems of the prior art described above and the object thereof is to provide an extrusion molding apparatus for a ceramic molded product in which lumps of an aggregated ceramic material can be passed through the filter unit smoothly.